The present invention relates to integrated circuits in general, and more specifically to a circuit used to protect against high voltages as may be used, for example, in programming or erasing a nonvolatile memory.
Non-volatile memories generally require a much higher voltage to write or erase a memory cell than is used as the main power supply to the chip. For example, the main supply voltage to the chip (Vdd) may be 3V or 5V, while an erase or programming operation requires application of 12-48V on the drain and/or [drain] of the transistor to be written. Charge pumps are typically employed to generate the higher voltages from the main power supply voltage. During a write operation, it is important that those memory cells which are not to be written are protected from the higher voltage. Furthermore, operation of the charge pumps can be quite sensitive to leakage such that protection circuits are sometimes needed to prevent such leakage.
One conventional protection circuit utilises two transistor in series, with the goal being that these transistors remain in their off state during application of the programming voltage. A simplified circuit diagram of such a protection circuit is shown in FIG. 1. A high voltage protection circuit 10 includes a first transistor 12 whose gate and source of transistor 12 are coupled to ground or Vss. The circuit also includes a second transistor 14 having its drain coupled to the high voltage being supplied (Vhv), its source coupled to the drain of transistor 12, and its gate coupled to a voltage labeled Vcas, because transistor 14 is also sometimes referred to as a cascode transistor. When Vhv is applied, second transistor 14 remains in an off state and shields transistor 12 from Vhv, which might otherwise be higher than the breakdown voltage of transistor 12. Second transistor 14 provides protection by ensuring that the voltage at the drain of first transistor 12 is below the breakdown voltage of transistor 12. This is ensured when the source of second transistor 14, and thus the drain of first transistor 12, does not rise above a voltage level equal to Vcas minus the threshold voltage (Vth) of second transistor 14.
Traditionally, Vcas was equal to Vdd, the main power supply voltage of the chip, because routing of this voltage to the protection circuit was very easy since the supply voltage is present in virtually every part of the chip. However, with the advancement of semiconductor manufacturing processes came the need for lower supply voltages (e.g.  less than 3V), such that using Vdd as Vcas no longer offered high voltage protection because second transistor 14 would prematurely breakdown, thereby exposing first transistor 12 to Vhv.
To improve the performance of protection circuit 10, Vcas can be raised to a voltage higher than Vdd but lower than Vhv. This intermediate voltage is sometimes referred to as a xe2x80x9cstand-offxe2x80x9d voltage (Vso) which is also generated from a charge pump. While Vso can be chosen to offer effective breakdown and high voltage protection, use of an intermediate voltage requires additional circuitry to generate Vso and additional routing to route Vso to all the protection transistors in the memory array. This additional circuitry and routing adversely affects chip size, which in turn increases manufacturing costs. Accordingly, it would be desirable to have a high voltage protection with minimum additional routing and circuitry which can be used in conjunction with integrated circuits having very low power supply voltages (e.g.  less than 3V).